1. Technical Field
The invention relates to electric motors and reduction gears and more particularly to a motorized gear reducer having improved characteristics.
2. Related Art
A motorized gear reducer is a combination of an electric motor and a reduction gear set. For tools or devices having the need of speed reduction, a motorized gear reducer can provide low speed and high torque output for saving labor or increasing thrust. For an electric bicycle equipped with a motorized gear reducer, its low speed and high torque output can save a rider's labor on an up-hill road. Details of an electric bicycle equipped with a motorized gear reducer can be found in U.S. Pat. Nos. 6,196,347, 6,296,072 and 6,629,574.
Typically, a motorized gear reducer with a reduction gear set is characterized in that a pinion is operatively connected to a motor shaft of a hub motor, and a gear (or a gear train) meshes with the pinion; or a worm gear is operatively connected to a motor shaft of a hub motor. Finally, a speed reduced output is made possible. Specifically, U.S. Pat. Nos. 6,196,347, 6,296,072 and 6,629,574 each discloses that a planetary gear train is operatively connected to a motor shaft of a hub motor for speed-reduced output. More specifically, U.S. Pat. No. 6,196,347 discloses a Ferguson's mechanical paradox gear in a motorized gear reducer. More specifically, U.S. Pat. Nos. 6,296,072 and 6,629,574 each discloses a two-stage gear reduction mechanism. However, all of above patents are bulky due to the excessive length of the motor shaft. Therefore they are not applicable to bicycles having limited longitudinal length.
For reducing the size of a motorized gear reducer, a planetary gear train pancake motor is developed (see http://www.haydonkerk.com/LinearActuatorProducets/StepperMotorLinearActuators/RotaryStepperMotors//PancakeRotaryMotors/PlanetaryPancakeMotors/tabid/107/Default.aspx#ballbearing). It discloses a rotor in a motor housing axially connected to a sun gear, other components of a planetary gear train disposed in a central area of the hollow type motor housing to achieve pancake motor design having one stage speed reduction. In detail, an internal gear (called annular gear or ring gear meshing with the planet gears at outer side), shaped as an outer ring, is located inside the rotor and secured to the motor housing. In short, it simply mounts a conventional planetary gear train in a motor housing for the purpose of decreasing length of the motor shaft. However, it does not decrease the volume of the planetary gear train pancake motor. To the worse, it adversely increases the number of components and greatly increases the manufacturing cost. Further, only a single stage speed reduction is possible.
U.S. Pat. Nos. 6,296,072 and 6,629,574 each discloses two planet gears meshing with two sun gears respectively for rendering a two-stage speed reduction mechanism. However, both patents do not teach a volume reduction of the motorized gear reducer.
One skilled in the art may use the disclosures of both U.S. Pat. Nos. 6,296,072 and 6,629,574 (i.e., two-stage speed reduction mechanism) to convert one-stage speed reduction in a motor housing of a planetary gear train pancake motor into a planetary gear train having the two-stage gear reduction mechanism. However, the following problems still exist: How to cause a motor rotor to drive planet gears or ring gear of a planetary gear train. How to mount a planetary gear train having a two-stage gear reduction mechanism in an electric motor within a limited space.
Thus, the need for improvement still exists.